The present proposal for preclinical studies with 12-O-tetradecanoylphorbol-13-acetate (TPA) will: 1. Identify and characterize combinations of TPA and other potential therapeutic agents that may enhance differentiation and therapeutic efficacy and overcome potential resistance to TPA-induced differentiation in the treatment of myeloid malignancies. 2. Examine the hypothesis that the effects of TPA, all-trans retinoic acid (ATRA), 1 alpha,25-dihydroxyvitamin D3 or butyrate alone or as synergistic combinations are mediated through changes in the levels and translocation of PKC alpha, PKC beta and PKC delta, and on the expression of mRNAs for PKC alpha, PKC beta and PKC delta in TPA-sensitive HL-60 and TPA-resistant HL-525 cells. We will also determine the effects of selective inhibitors of PKC beta and PKC delta as well as the expression of dominant negative PKC isoforms in HL-60 cells on the differentiation response of these cells to TPA alone or together with potential synergistic agents. The effects of overexpression of constitutive PKC isoforms on the action of TPA alone to cause differentiation in TPA-resistant HL-525 cells will also be determined. 3. Study the hypothesis that TPA may have therapeutic efficacy for inducing cytotoxicity in human prostate cancer cells. Specifically, we will determine the effects of TPA alone or in combination with (a) differentiating agents (ATRA, 1alpha, 25-dihydroxyvitamin D3, and butyrate), (b) inhibitors of NF-kappaB (BAY 11-7082) or (c) cytotoxic drugs (taxol, taxotere, mitoxantrone and estramustine) on proliferation and apoptosis of cultured human prostate cancer cell lines. We will test the hypothesis that levels of constitutive NF-kappaB activity determine sensitivity of these cells to TPA-induced cytotoxicity. The effects of TPA alone or in combination with synergizers on the levels and translocation of PKC alpha PKC beta and PKC delta and on the expression of these genes in prostate cancer cells will be evaluated. 4. Determine the in vivo effects of TPA alone, or in combination with other agents that are synergistic, on the growth of human prostate tumors in immunodeficient mice. Blood levels of TPA that are associated with inhibition of tumor growth will be determined. The effect of inhibitors of tumor growth on proliferation and apoptosis in the tumors will be determined.